Plasma membrane is studded with a variety of membrane proteins that act as ion channels. Each channel only allows certain types of ions to pass across the membrane. Most channels are specific (selective) for one ion. The channel pore is typically so small that ions must pass through it in single file.
A channel may have several different states (corresponding to different conformations of the protein), but each such state is either open or closed. In general, closed states correspond either to a contraction of the pore—making it impassable to the ion—or to a separate part of the protein, stoppering the pore. When a channel is open, ions permeate through the channel pore down the transmembrane concentration gradient for that particular ion. Rate of ionic flow through the channel is determined by the maximum channel conductance and electrochemical driving force for that ion.
Ion channels can be classified by how they respond to their environment. For example, ion channels can be voltage-sensitive in that they open and close in response to the voltage across the membrane. Ligand-gated channels form another important class; these ion channels open and close in response to the binding of a ligand molecule such as a neurotransmitter. Other ion channels open and close with mechanical forces. Still others, such as those of sensory neurons, open and close in response to other stimuli, such as light, temperature, or pressure. The most common types of ion channels are described below.
Leakage channels are the simplest type of ion channel, in that their permeability is more or less constant. The types of leakage channels with the greatest significance in neurons are potassium and chloride channels. Although they are the simplest in theory, most conduct better in one direction than the other (they are rectifiers) and some are capable of being shut off by ligands even though they do not require ligands in order to operate.
There are three main types of gated channels: chemically-gated or ligand-gated channels, voltage-gated channels, and mechanically-gated channels.
Ligand-gated ion channels are channels whose permeability is greatly increased when some type of chemical ligand binds to the protein structure. A large subset function as neurotransmitter receptors—they occur at postsynaptic sites, and the chemical ligand that gates them is released by the presynaptic axon terminal. Ligand-gated channels can be activated by ligands that appear in the extracellular area or by interactions on the intracellular side.
Voltage-gated ion channels, also known as voltage-dependent ion channels, are channels whose permeability is influenced by the membrane potential. They form another very large group, with each member having a particular ion selectivity and a particular voltage dependence. Many are also time-dependent—in other words, they do not respond immediately to a voltage change, but only after a delay. Voltage-gated channels are essential for the generation and propagation of action potentials.
Ion pumps are not ion channels, but are critical membrane proteins that carry out active transport by using cellular energy (ATP) to "pump" the ions against their concentration gradient. Such ion pumps take in ions from one side of the membrane (decreasing its concentration there) and release them on the other side (increasing its concentration there).